Biological Clocks
Biological Clocks,
physiological systems that enable organisms to live in harmony with the
rhythms of nature, such as the cycles of day and night and of the seasons.
Such biological "timers" exist for almost every kind of periodicity
throughout the plant and animal world, but most of what is known about them
comes from the study of circadian, or daily, rhythms. Circadian rhythms cue
typical daily behavior patterns even in the absence of external cues such as
sunrise, demonstrating that such patterns depend on internal timers for their
periodicity.
No clock is perfect, however. When organisms are deprived of
the cues the world normally provides, they display a characteristic
"free-running" period of not quite 24 hours. As a result,
free-running animals drift slowly out of phase with the natural world. In
experiments in which people are isolated for long periods of time, they
continue to eat and sleep on regular, but increasingly out-of-phase,
schedules. Such drift does not take place under normal circumstances, because
external cues reset the clocks each day.
Light, particularly bright light, is believed to be the most
powerful synchronizer of circadian rhythms. Recent studies on humans have
shown that the amount of artificial indoor light to which people are exposed
per day can resynchronize the body's cycle of sleep and wakefulness. People
can inadvertently reset their body clocks to an undesired cycle by such
activities as shielding morning light with shades and heavy curtains or by
reading in bed at night by bright lamp light. Many organisms also make use of
rhythmic variations in temperature or other sensory inputs to readjust their
internal timers. When a clock's error becomes large, complete resetting
sometimes requires days. This phenomenon is well known to long-distance air
travelers as jet lag.
Apparently, biological clocks can exist in every cell and even
in different parts of a cell. Hence, an isolated piece of tissue removed from
an organism—for example, the eye of a sea slug—will maintain its own daily
rhythm but will quickly adopt that of the whole organism when restored to it.
In the brains of most animals, a master clock appears to exist
that communicates its timing signals chemically to the rest of the organism.
For example, a brain removed from a moth pupa and exposed to an artificial
sunrise of one time zone, then implanted into the abdomen of a headless pupa
on a different time zone schedule, will cause the second pupa to emerge at the
time of day appropriate to the disconnected brain floating in its abdomen. The
clock in the brain triggers the release of a hormone that switches on all the
complex behavior involved in pupal emergence. In hamsters, experiments have
shown a master biological clock to be located in the hypothalamus.
Scientists believe that the biological clock in humans is
located in the hypothalamus, the part of the brain that regulates such basic
drives as hunger, thirst, and sexual desire. The biological clock itself is
believed to be a cluster of nerve cells called the suprachiasmatic nucleus.
Melatonin, a hormone produced by the pineal gland in response
to darkness, is thought to play a primary role in controlling the body's
circadian rhythm. Recent studies have found that very low doses of melatonin,
administered as a supplement, can induce sleep, making the hormone potentially
useful as a remedy for sleep disorders or jet lag.
Recent biochemical studies on fruit flies, as well as earlier
research on bread mold, have discovered genes that play an important role in
the biological clocks of these organisms. In bread mold, a gene known as freq
has been shown to be integral to the mold's biological clock. In the fruit
fly, two proteins—encoded by two genes known as per (for period)
and tim (for timeless)—appear to interact together with light
to govern the insect's biological clock. How the fruit fly's clock mechanism
relates to that of humans is not clear because neither tim nor per
has been found in mammals.
A fuller understanding of biological clocks could be important
in many ways. One promising theory of aging, for example, is based on an
observation that, in old age, the many separate, subordinate clocks in the
body seem somehow to become less tightly coupled to the master clock in the
brain. This lack of synchronization may contribute to many of the problems
associated with aging.
